I do not use adsense but I do use Adwords. Using Velaro which I use to monitor my visitors I have noticed that all visitors that come from Adwords now have a client ID added to the end of the URL. I don't know if this means anything but thought I should let people know.. It is a wired looking code and every one is different.
they use to be one step ahead now in detecting click fraud, as far as i know, client ID will be created using cookies, different PC, diff cookies, that's mean they will be able to filter out either publisher is clicking it's own ads, that come from same cookie, if then you clear your cookie, then you can start again, but what about diff ip?? they also evaluate click fraud based on ip, and what about mass bot that comes infected to every PC in the world, different ip and cookies, that will be another story, I've read somewhere about that, advertisers lose very much money.